Btooom. I haven't seen SAO and Fate/Zero, and Highschool of the Dead is overloaded with fanservice. Admittedly, Btooom also has fanservice, but not much.

Actually, I would rank Btooom! second with the most fanservice among these 4 anime.

1. HOTD
2. Btooom!
3. SAO
4. Fate

Is this a ranking of fanservice because if so Fate/Zero should be at the top Fate/Stay Night Archer has nothing on Gilgamesh

Spoiler Alert! Click to show or hide

How could you say Fate on the top?
HOTD even showed nipples in the bath scene, you know.

Btooom! : Rape scene! A few sexy poses!
SAO: mm... A squeeze on Asuna! Rape scene on world tree!

Fate:
Come on, he is male, and his exposed body was just like a normal male character plays around at the beach.

Well I was mostly being sarcastic, so it's my fault for not inflecting. Fate/Zero had little to no fanservice, only notable moment was with Gils. No my friend King of Heroes easily trumps those pathetic mongrel's. Also there is such thing as fanservice that doesn't involve women